This invention pertains to an improved process for heat recovery in gasifying solid carbonaceous fuels such as coal to provide a clean gaseous fuel product. It pertains particularly to such gasification process using a lower fluidized bed for devolatilization and combustion of a coal feed and an upper fluidized bed for recovery of heat from gasifier effluent, with the beds separated by an intermediate gas-solids disengaging section.
For high temperature fluidized or entrained bed type coal gasifiers operating at 1500.degree. F. and higher outlet temperatures, the dust carried off from the gasifier usually contains sticky particles which will deposit on surfaces within dust recovery cyclone separators and other downstream equipment and seriously interfere with their operation. The sticky particles are primarily due to the iron sulfide and/or oxide complex compounds from the ash portion of the char fines. Low melting point eutectic compounds from vanadium, sodium, potassium, calcium and others may also form sticky particles on the equipment. The U.S. eastern high sulfur bituminous coals are particularly prone to such ash deposit problems. The prior art discloses that overcoming this problem is accomplished either by quenching the gasifier effluent before it leaves the unit, or by contacting the effluent in a cold wall equipment to solidify the sticky ash. However, these methods have the disadvantage in that a large portion of the high level heat is undesirably lost. For example, in quenching the effluent from 1850.degree. F. to 1500.degree. F. approximately 25% of the high level heat is lost in vaporizing the quench water.
Also in a gasifier that is subject to gasifier effluent load variations, the design of the waste heat recovery system can be complicated. The heat recovery system surfaces, such as shell and tube heat exchangers, must be arranged in such a manner between steam generation and steam superheating to permit adequate flexibility. The invention described herein overcomes these problems.
The gasification of coal using a lower fluidized bed gasification zone and a superposed fluidized bed cooling zone is generally disclosed by U.S. Pat. No. 2,591,595 to Ogorzaly. Also, U.S. Pat. No. 4,099,933 to Johnson et al. discloses the gasification of coal using multiple superposed zones maintained at desired temperature ranges to control the coal heating rate, particularly for caking type coals. However, further improvements in coal gasification are desired, particularly to eliminate sticky particles in the product gas and to provide for more efficient heat recovery.